Lithium-ion secondary batteries as storage batteries (hereinafter, lithium ion batteries), which have a large energy density and excellent charge and discharge cycle characteristics, are widely used mainly for electronic equipment such as portable devices. Here, lithium ion batteries are broadly classified into: (1) electrolyte solution-based lithium ion batteries and (2) all-solid lithium ion batteries according to the type of electrolyte solution to be used.
(1) Electrolyte Solution-Based Lithium Ion Batteries
As shown in FIG. 1, an electrolyte solution-based lithium ion battery, which is the most common lithium ion battery used widely at the present day, comprises the basic elements of a positive electrode (a positive-electrode current collector 1, a positive-electrode active-material layer 2), a negative electrode (a negative-electrode current collector 6, a negative-electrode active-material layer 5), an electrolyte solution 4, and a separator 3. It has a cell structure covered with a container 8 (metal, plastic laminate, and the like) in which the positive electrode and the negative electrode each having an electrode lead 7 (extraction electrode) attached thereto are immersed in the electrolyte solution 4 in a state where the separator 3 capable of retaining an electrolyte between the electrodes is present between the electrodes.
The above positive electrode and the above negative electrode have a structure in which a corresponding active-material layer (the positive-electrode active-material layer 2 or the negative-electrode active-material layer 5) comprising crystalline particles of a corresponding active material (a positive-electrode active material or a negative-electrode active material), an electroconductive auxiliary agent, a binding material (binder) and the like as main components is formed on a corresponding current collector (the positive-electrode current collector 1: aluminum foil and the like, or the negative-electrode current collector 6: a copper foil and the like).
A composite oxide comprising lithium and a transition metal is usually used as the above positive-electrode active material. Specifically, commonly used are lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), a lithium-nickel-cobalt-aluminum oxide (Li(Ni—Co—Al)O2), a lithium-nickel-manganese-cobalt oxide (LiNi1/3Mn1/3Co1/3O2) as a layered-based material, a lithium-manganese oxide (LiMn2O4) as a spinel-based material, and lithium iron phosphate (LiFePO4) as an olivine-based material and like. Further, the following are also under development for achieving higher energy: a lithium-manganese-nickel oxide (Li(Mn3/2Ni1/2)O4 and the like) as a spinel-based material for performing charge and discharge at a high voltage (in the range of 5 V), a solid solution-based (also referred to as “excess-based”) manganese-containing lithium composite oxide (for example, Li2MnO3—LiMO2 [M:Ni, Mn, Co and the like) as a layered-based material having a high capacity. Moreover, as the above negative-electrode active material, black lead (graphite), lithium titanate (Li4Ti5O12) and the like are generally used.
Meanwhile, a corresponding active-material layer (the positive-electrode active-material layer 2 or the negative-electrode active-material layer 5) can be formed on a current collector by applying and drying (and press-densifying, if desired) a paste for forming a corresponding active-material layer (a paste for forming a positive-electrode active-material layer or a paste for forming a negative-electrode active-material layer).
Here, as a paste for forming a positive-electrode active-material layer, commonly used is, for example, a nonaqueous paste in which particles of the positive-electrode active material and particles of an electroconductive auxiliary agent such as acetylene black (AB) and vapor-grown carbon fiber (VGCF) are dispersed in an organic solvent such as N-methylpyrrolidone (NMP) in which a binding material (binder) such as poly(vinylidene fluoride) (PVDF) is dissolved. Further, as a paste for forming a negative-electrode active-material layer, commonly used is, for example, an aqueous paste in which particles of the negative-electrode active material and, if desired, particles of an electroconductive auxiliary agent such as acetylene black (AB) and vapor-grown carbon fiber (VGCF) are dispersed in water containing styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA) and the like as an aqueous binding material (binder).
In the above electrolyte solution-based lithium ion battery, various improvements have been continually made for an active material (a positive-electrode active material and a negative-electrode active material), an electrolyte solution, a separator and the like in order to improve performance thereof and to reduce cost thereof. However, an electrolyte solution-based lithium ion battery using a liquid electrolyte solution as an electrolyte poses a fire risk due to a flammable organic solvent being used as a main component of the electrolyte solution. Further, an electrolyte solution may also leak. Therefore, the electrolyte solution-based lithium ion battery cannot be considered sufficiently safe. Furthermore, the capacity thereof is difficult to increase by increasing voltage because an electrolyte solution usually undergoes decomposition at a cell voltage of about 4.5 V or more. In addition, a smaller and higher-voltage cell is difficult to achieve by using a bipolar layered cell structure because cells in a container (package) are electrically connected to each other via an electrolyte solution. For the above reasons and the like, electrolyte solution-based lithium ion batteries using a liquid electrolyte solution as an electrolyte have limitations for further reducing cost and achieving smaller sizes and better performance. Accordingly, attempts have actively been made in recent years for developing an all-solid lithium ion battery in which a solid electrolyte is used instead of an electrolyte solution.
(2) All-Solid Lithium Ion Battery
An all-solid lithium ion battery, which is characterized in that a nonflammable/flame-retardant solid electrolyte is used instead of an inflammable electrolyte solution, has the following advantages. Leakage of an electrolyte solution from a cell and ignition can be prevented, leading to significantly improved safety. In addition, the above bipolar layered cell structure can allow a package to be thinner/smaller and a higher cell voltage to be obtained by having in-series (layered) connections within a single cell, leading to lower cost. Moreover, a higher capacity can further be achieved by using a positive-electrode active material (the 5-V system) with a high potential versus Li because a potential window of a solid electrolyte can be broadened.
As shown in FIG. 2, the basic cell structure of an all-solid lithium ion battery is a layered cell structure in which current collectors (the positive-electrode current collector 1 and the negative-electrode current collector 6) and functional layers (the positive-electrode active-material layer 2, a solid-electrolyte layer 9, and the negative-electrode active-material layer 5) are layered. By layering two or more of these layered cell structures, a bipolar layered cell structure can easily be obtained (n layers: a current collector/a positive-electrode active-material layer (first layer)/a solid-electrolyte layer (first layer)/a negative-electrode active-material layer (first layer)/a current collector/a positive-electrode active-material layer (second layer)/a solid-electrolyte layer (second layer)/a negative-electrode active-material layer (second layer)/a current collector/ . . . /a positive-electrode active-material layer (nth layer)/a solid electrolyte layer (nth layer)/a negative-electrode active-material layer (nth layer)/a current collector). The electrode lead 7 (extraction electrode) is attached to each of the outermost current collectors of the layered cell structure (the positive-electrode current collector 1, the negative-electrode current collector 6), and covered with the container 8 (a metal, a laminated plastic and the like). As the above current collector, a stainless steel foil, an aluminum foil, or the like can be used for the positive-electrode current collector 1, and a stainless steel foil, a copper foil, or the like can be used for the negative-electrode current collector 6.
Further, in the above functional layers (the positive-electrode active-material layer 2, the solid-electrolyte layer 9, and the negative-electrode active-material layer 5), in an active-material layer (the positive-electrode active-material layer 2 or the negative-electrode active-material layer 5) for example, crystalline particles of a corresponding active material (a positive-electrode active material or a negative-electrode active material) are dispersed in a solid electrolyte matrix, and a trace amount of an electroconductive auxiliary agent for improving current-collecting properties (electron conductivity) within the active-material layer and a binding material (binder) for conferring adhesiveness and plasticity may be contained if desired. Moreover, the solid-electrolyte layer 9 is a polycrystalline layer in which solid-electrolyte particles are densely packed, or is a dense noncrystalline layer consisting of the solid electrolyte, and may comprise a trace amount of a binding material (binder) for conferring adhesiveness and plasticity, if desired.
For the above active materials (positive-electrode active material, negative-electrode active material), materials similar to those for active materials used in the aforementioned electrolyte solution-based lithium ion battery can be used. Furthermore, advantageously, a high-capacity negative-electrode active material such as metal lithium or various lithium alloys, which cannot be used for an electrolyte solution-based lithium ion battery because metal lithium undergoes dendrite deposition at a negative electrode and causes short circuiting, can also be used for an all-solid lithium ion battery.
Incidentally, solid electrolytes used for all-solid lithium ion batteries can be broadly classified into oxide-based solid electrolytes and sulfide-based solid electrolytes. For example, oxide-based solid electrolytes include lithium phosphate (Li3PO4), Li3PO4Nx in which nitrogen is added to lithium phosphate (referred to as LiPON), LiBO2Nx, LiNbO3, LiTaO3, Li2SiO3, Li4SiO4—Li3PO4, Li4SiO4—Li3VO4, Li2O—B2O3—P2O5, Li2O—SiO2, Li2O—B2O3—ZnO, Li1+XAlXTi2-x(PO4)3 (0≦X≦1) (referred to as LATP or LTAP), Li1+XAlXG2-X(PO4)3 (0≦X≦1; specifically Li1.5Al0.5Ge1.5(PO4)3) (referred to as LAGP), LiTi2(PO4)3, Li3XLa2/3-XTiO3 (0≦X≦⅔; specifically Li0.33La0.56TiO3, Li0.5La0.5TiO3 and the like) (referred to as LLT or LLTO), Li5La3Ta2O12, Li7La3Zr2O12 (referred to as LLZ or LLZO), Li6BaLa2Ta2O12, Li3.6Si0.6P0.4O4 and the like. Sulfide-based solid electrolytes include Li2S—SiS2, LiI—Li2S—SiS2, LiI—Li2S—P2S5, LiI—Li2S—B2S3, Li3PO4—Li2S—Si2S, Li3PO4—Li2S—SiS2, LiPO4—Li2S—SiS, LiI—Li2S—P2O5, LiI—Li3PO4—P2S5, Li2S—P2S5 and the like. However, sulfide-based solid electrolytes, which have a higher conductivity for lithium ions than oxide-based solid electrolytes, are preferable. Among these, an Li2S—P2S5-based solid electrolyte is preferred in view of excellent properties and cost reduction (expensive metal elements are not included).
It is known that when oxide particles having excellent electron conductivity (for example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), a lithium-nickel-cobalt-aluminum oxide (Li(Ni—Co—Al)O2), a lithium-nickel-manganese-cobalt oxide (LiNi1/3Mn1/3Co1/3O2 and the like), oxide particles having moderate electron conductivity (a lithium-manganese-nickel oxide (Li(Mn3/2Ni1/2)O4 and the like), a solid solution-based (excess-based) manganese-containing lithium composite oxide (Li2MnO3—LiMO2 [M:Ni, Mn, Co] and the like)) are used as an active material for an all-solid lithium ion battery in which the above sulfide-based solid electrolytes are used, a lithium ion-deficient space-charge layer is formed in the side of the sulfide-based solid electrolyte at the interface of the active-material particles (oxide particles) and the sulfide-based solid electrolyte in an active-material layer, resulting in significantly deteriorated output characteristics (rate characteristics) of a battery due to significantly increased resistance at the interface. Accordingly, as a method of effectively reducing interface resistance, a coating of active-material particles (oxide particles) with an oxide solid electrolyte having low electron conductivity (for example, an alkali composite oxide such as Li2SiO3, LiNbO3, Li4Ti5O12, Li2Ti2O5 and the like) is proposed and widely tested (see Patent Literatures 1 to 3 and the like).
Although lithium ion batteries as storage batteries are described above, attempts have actively been made in recent years at developing a low-cost next-generation battery with a higher capacity (the next generation secondary battery) as a storage battery, for example a sodium ion battery or a magnesium ion battery. Even in these batteries, basic structures thereof are similar to those of the aforementioned lithium ion batteries, but sodium or magnesium is used instead of lithium.                Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2009-266728        Patent Document 2: Pamphlet of PCT International Publication No. WO2013/011871        Patent Document 3: Pamphlet of PCT International Publication No. WO2013/046443        